Just for clarification, do you mean using bread to thicken a sauce? If so, only a little is needed. I believe it is Susan Spicer who makes an incredible garlic soup using bread pureed with it to give the soup body.

There is a good recipe on the jean George's book from the early 90s. He uses the crumbs from the days bread cutting, toasts and infuses water and vinegar purée then mount with butter if I remember correctly

Method

Put the white bread in a food processor and blitz to fine breadcrumbs. Weigh out 55g and discard the excess.

Stud the onion with the cloves, then put it in a small saucepan with the milk, bay leaf, peppercorns and nutmeg. Put the pan over a medium-low heat.

Once the milk is warm, remove the pan from the heat and allow the milk to infuse for 20 minutes. Strain, return to the pan and add the breadcrumbs, butter and cream. Warm gently over a medium heat for 5 minutes, stirring occasionally, until thickened. Season with salt before serving.